The invention relates to power production units of the type comprising a free piston gas generator having a plurality of compression chambers and at least one motor cylinder as well as a gas turbine which receives the exhaust gas from the combustion chambers of the motor cylinder(s) and/or pressurized gas from the compression chambers and which may drive an electric generator.
A power unit of that type is described in Applicant's earlier European Patent No. 0007 874. The compression chambers of that power unit are distributed into two sets. The compression chambers of the first set deliver hot pressurized air to an auxiliary turbine which drives a rotary compressor for supercharging all compression chambers through cooling means. The motor cylinder(s) receive(s) pressurized air from the other set of compression chambers and their combustion gases are directed to a power gas turbine which also receives part of the hot air from the first set.
The arrangement has the advantage that use is made of the hot air flow delivered by the compression chambers and in excess of that necessary for efficient operation of the motor cylinders, (whose working cycle is somewhat comparable to that of a two stroke Diesel engine) under acceptable conditions. However, that air which is delivered by the first set is at a temperature substantially lower than the exhaust gas. For avoiding thermal problems at the output of the compression chambers, it is of advantage to increase the rate of flow--and consequently to decrease the output temperature--rather than the pressure. Mixing of gas and hot air at different temperatures is far from an optimum.
It is an object of the invention to provide a power unit of the above type which has an increased efficiency without requiring high gas temperatures which detrimentally affect the components subjected to the action of hot gas. It is a more specific object to provide a power unit which associates the high air compression efficiency of alternating machines and the high degree of efficiency and long life of gas turbines.
For that purpose, there is provided a power unit comprising a free piston gas generator having a plurality of compression chambers, motor cylinder means, and a reciprocable assembly having drive piston means slidably received in said motor cylinder means and a plurality of compression pistons cooperating with said compression chambers, whereby reciprocation of said assembly alternately increases and decreases the volumes of said compression chambers and a power gas turbine connected to receive a flow of exhaust gas from an exhaust of said motor cylinder means. The unit further comprises a rotary compressor connected to the compression chambers through first cooling means to deliver air at above atmospheric pressure to the compressor chambers, which are distributed into a first set and a second set. The outlets of the first set of compression chambers are connected to deliver air to the motor cylinder means through second cooling means, the outlets of the second set are connected to deliver hot pressurized air to the flow of exhaust gas prior to admission to said power gas turbine and the gas mixture is delivered to the turbine where it expands. Heating means are provided for increasing the temperature of the mixture of hot air and exhaust gas before delivery to the turbine. They include heat exchange means for heat exchange between said hot pressurized air and gas which has expanded in the gas turbine and/or a combustion chamber. The rotary compressor may preferably be driven by an auxiliary gas turbine connected to receive gas from the outlet of the power turbine and to deliver that gas to the heat exchanger means. The combustion chamber is typically dimensioned for the thermal power developed by said chamber to be of the same order of magnitude as that of the motor cylinders.
In a particular embodiment, the unit may comprise an additional high pressure loop having a rotary HP compressor connected to receive air from the second set of compression chambers and a rotary HP turbine for driving the HP compressor. The gas expanded in the HP turbine mixes with the flow of exhaust gas. A second combustion chamber is then located in the loop between the HP compressor and the HP turbine. Then, the thermal energy added by the heating means will result into a pressure increase and after expansion in the HP gas turbine. For decreasing that pressure to a level equal to that of the exhaust gas from the motor cylinders, the unit may include a gas pressure balancing turbine connected to receive the gas from the HP turbine and to reduce its pressure to a value substantially equal to that of the exhaust gas prior to mixing.
The invention will be better understood from the following description of preferred embodiments of the invention, given by way of examples only.